In exploring the developments of electric power converters that exhibit lower electric power consumption, power devices that play a key role in the electric power converters have been expected to exhibit a lower electric power consumption. Among the power devices, insulated gate bipolar transistors (hereinafter referred to as “IGBTs”) have been popular because the IGBTs can achieve a low ON-state voltage drop due their conductivity modulation effects and can be driven easily via their gate controlled with a voltage applied thereto. In particular, trench IGBTs, which have a trench structure including trenches formed from the wafer surface and gate electrodes buried in the respective trenches with an oxide film interposed between the gate electrode and the trench wall, are now becoming more popular. Since channels are formed on both sides of each trench, the trench IGBT increases the channel density and lowers the ON-state voltage drop thereof as compared with the so-called planar IGBTs that have gate electrodes on the wafer surface.
Referring to FIG. 14, which show a cross section of a conventional n-channel IGBT cut in perpendicular to the stripe-shaped trenches, p-type base regions 3 are formed in the surface portions of a silicon wafer formed of a heavily doped p-type silicon substrate 1 and a lightly doped n-type drift layer 2. In the surface portions of the p-type base region 3, n+-type emitter regions 4 are selectively formed. A trench is formed from the surface of the n+-type emitter region 4 down to the n-type drift layer 2, through the p-type base region 3. A polysilicon gate electrode 6 is formed in the trench with a gate oxide film 5 interposed between the gate electrode 6 and the trench wall. An interlayer insulator film 7 is formed to cover the upper portion of gate electrode 6. A sheet shaped emitter electrode 8 is formed on the interlayer insulator film 7 such that the emitter electrode 8 is in contact commonly with the n+-type emitter regions 4 and the p-type base regions 3. Although a passivation film, such as a nitride film and an amorphous silicon film, can be formed on the emitter electrode 8, the passivation film is not shown in FIG. 14. A collector electrode 9 is formed on the back surface of the p-type silicon substrate 1.
The trench IGBT is brought into its ON-state as follows. The IGBT is in the OFF-state thereof when the voltage of the gate electrode 6 is lower than the threshold value in the ON-state. The emitter electrode 8 is typically grounded and a voltage higher than the voltage of the emitter electrode 8 is applied to the collector electrode 9. As a voltage higher than the threshold value is applied from a gate driver circuit to the gate electrode 6 through a gate resistance, electric charges start accumulating in the gate electrode 6. Simultaneously with the charge accumulation in the gate electrode 6, the portion of the p-type base region 3 facing the gate electrode 6 via the gate oxide film 5 is inverted to an n-type to form a channel region. As the channel regions are formed, electrons are injected from the emitter electrode 8 to the n-type drift layer 2 via the n+-type emitter regions 4 and the p-type base regions 3. The injected electrons bias the p-type silicon substrate 1 and the n-type drift layer 2 forwardly and holes are injected from the collector electrode 9, resulting in an ON-state of the IGBT. The voltage drop between the emitter electrode 8 and the collector electrode 9 is the ON-state voltage drop.
To bring the IGBT from the ON-state to the OFF state, the electric charges accumulated in the gate electrode 6 are discharged via the gate resistance to the gate diver circuit by setting the voltage between the emitter electrode 8 and the gate electrode 6 to be lower than the threshold value. As the electric charges accumulated in the gate electrode 6 are discharged, the channel regions inverted to the n-type return to the p-type. Since the channel regions vanish, the electron supply is stopped. Since the hole supply is also stopped, the electrons and the holes accumulated in the n-type drift layer 2 are ejected to the collector electrode 9 and the emitter electrode 8 respectively, or the current vanishes due to the recombination of the electrons and the holes, bringing the IGBT into the OFF-state thereof.
Various improvements have been proposed to further reduce the ON-state voltage drop of the trench IGBT. The injection enhanced gate bipolar transistor (hereinafter referred to as the “IEGT”) disclosed in the Japanese patent publication JP P Hei. 5 (1993)-243561 A exhibits the ultimate characteristics close to the ON-state voltage drop of the diode. The IEGT covers part of the surfaces of the n+-type emitter regions and the p-type base regions with an insulator film so that the covered regions and the emitter electrode do not contact each other. Although the operation of the IEGT is fundamentally the same as that of the trench IGBT, the holes, below the p-type base region in the portion where the n+-type emitter regions and the p-type base region are not in contact with the emitter electrode, are hardly ejected to the emitter electrode. The holes, being hardly ejected to the emitter electrode, accumulate and the carrier concentration distributions in the n-type drift layer becomes close to the carrier concentration distributions in the diode. Therefore, the IEGT can reduce the ON-state voltage drop thereof lower than the ON-state voltage drop of the typical trench IGBT. However, the power devices also need to exhibit a high-speed switching performance in addition to the low ON-state voltage drop. Therefore, it is also important to improve the high-speed switching performance of the power devices. But large capacitance is formed between the gate electrodes and the emitter electrode in the trench IGBT and the IEGT, since trench structures are formed very densely in the trench IGBT and the IEGT.
As described above in connection with the operations of the IGBT shown in FIG. 14, it is necessary to charge and discharge to the capacitance between the gate electrodes and the emitter electrode in the transition from the ON-state to the OFF-state and vice versa. When the capacitance between the gate electrodes and the emitter electrode is large, the period for charging and discharging increases. The losses in the power device include the steady state losses determined by the ON-state voltage drop and the switching losses caused by the ON-OFF operations. Therefore, it is important to reduce the capacitance between the gate electrodes and the emitter electrode that causes the switching losses.
A structure similar to that shown in FIG. 14 is disclosed in JP P 2001-308327 A. By disposing a region. 11 covered with the insulator film 7 and not in contact with the emitter electrode 8, the holes, which hardly eject to the emitter electrode 8, become accumulated in the region 11 so that the carrier concentration distributions in the n-type drift layer can be close to the carrier concentration distributions in the diode. Moreover, since no trench gate structure covered with the insulator film 7 and not working for a control electrode, is formed in the region 11, the capacitance between the gate electrodes and the emitter electrode is reduced, the charging period and the discharging period are shortened, and the switching losses are reduced.
The structure disclosed in JP P 2001-308327 A, however, includes a floating mesa region 11. As described in Yamaguchi, et. al., “IEGT Design Criterion for Reducing EMI Noise”, in Proc. ISPSD' 2004, pp. 115-119, (2004), commonly on the structures disclosed in the above identified Japanese patent publications, there still remains room for improving the turn-on characteristics of the structures thereof.
It is essentially difficult for the structure shown in FIG. 14 to obtain a high breakdown voltage. Since the trenches are arranged at unequal intervals, the electric field is distributed unevenly and liable to localize to the bottom potions of the trench gates. In view of the foregoing, it would be desirable to provide an insulated gate semiconductor device that can suppress the ON-state voltage drop of the trench IGBT as low as the ON-state voltage drop of the IEGT, while reducing the switching losses, reducing the total losses, improving the turn-on characteristics, and obtaining a high breakdown voltage. The present invention addresses this need.